It was as thick as a cherry tomato, but it sounded like a battle shell. On June 26th, a fragment of an old space rock tore across the heavens of the southeastern United States at 29,000 miles per hour, rattling windows with a sonic boom and generating a chain of false alarms for an earthquake. The subsequent minutes saw it blow the roof from a McDonough, Georgia house, shred an air conditioning duct, and clang the floor killing part of itself into tiny dust the owner still keeps in his living room. University of Georgia researchers quickly recognized the unusual nature of what they had been given: 23 grams of the 50 grams of rescued meteorite from the crash.
Scott Harris, a geologist of planet science at Franklin College of Arts and Sciences at UGA, oversaw the research. “This particular meteor that entered the atmosphere has a long history before it made it to the ground of McDonough, and in order to totally understand that, we actually have to examine what the rock is and determine what group of asteroids it belongs to,” Harris stated. Determining that was fleeting.
Using optical and electron microscopy, the scientists identified the sample as a low-metal (L) ordinary chondrite, one of the stony meteorites that make up the majority of reported falls. Chondrites are among the solar system’s most primitive objects, maintaining the early protoplanetary disk’s mineral and chemical composition. This one, Harris discovered, was formed around 4.56 billion years ago, roughly 20 million years before the planet itself. In its wake, scientists were led a long way out into the asteroid belt between Jupiter and Mars.
Harris linked the McDonough Meteorite to a clan of bodies created by a cataclysmic break-up 470 million years ago an event that formed hundreds of thousands of other L chondrites too, which, according to studies on fossil meteorites, used to dominate the delivery of extraterrestrial material to Earth. In this very ancient influence, material was hurled into new orbits; some, within millions of years, evolved into Earth-crossing orbits. The McDonough fragment orbit became one that intersected with that of Earth at the wrong time for one Georgia resident. Its arrival was a perfect exercise in meteoroid atmospheric entry physics.
Upon entering the atmosphere, the object was traveling faster than the speed of sound many times. “You’re talking about something that is double the size of a 50-caliber shell, going at least 1 kilometer per second. That’s like running 10 football fields in one second,” Harris said. Though the atmosphere of Earth slowed it down immensely, the rest of the kinetic energy was enough to create a sonic boom and devastating impact. The boom, a shock wave of hypersonic entry, is a diagnostic of value to scientists, whose pressure profile and frequency content tell us about entry velocity, diameter, and altitude techniques learned in impact analysis of such occurrences as the Carancas meteorite. At McDonough, Harris thinks the homeowner was lucky to have heard three nearly simultaneous acoustical events: the roof struck, the sonic boom’s “tiny cone” and floor impact. The break-up of the impact was so complete that the part which remained intact was splinters; the rest was airborne dust, which fell silently on the room.
This kind of meteorite is rare in Georgia this was the 27th fall documented in the state, and the sixth observed as it fell.
However, as Harris explained, these recoveries are becoming more and more ordinary. Advances in observing networks, infrasound detection, and public education are making recovery more probable. This is more than an issue of ownership interest: every recovered meteorite is a genuine sample from somewhere else in the solar system with isotopic and mineralogical signatures of planetary evolution. The McDonough Meteorite will be left at UGA for ongoing research, with other fragments going on public display at the Tellus Science Museum in Cartersville.
Harris is preparing a full report on its composition, entry forces, and orbital record, work also including planetary defense planning. One day there will be an opportunity, and we never know when it’s going to be, for something large to hit and create a catastrophic situation. If we can guard against that, we want to, he said.
